Recently it has been demonstrated that covalent bonded ceramics, such as silicon carbides (SiC, SiOC), silicone nitrides, (Si3N4, SiCN), aluminum nitride (AlN) and hafnium carbide (HfC) can be synthesized by thermal decomposition of suitable polymeric precursors, and are referred to as polymer derived ceramics. Two of the inventors have previously made application for a patent on related work entitled ‘Method For Synthesizing Bulk Ceramic And Structures From Polymeric Ceramic Precursors’ (U.S. Patent Application 61/029,651). That patent application refers to a method for creating ‘bulk ceramic’ . . . which is a ceramic which has a high density.
When PDCs are polymerized from their liquid state, they shrink slightly, which can introduce stresses between them and objects incorporated in them, or onto which they are applied. When the polymerized PDCs are pyrolyzed, they undergo significant shrinkage and can deform, separate and fracture, if not defined and supported properly.
Other inventors, such as Chen et al, (U.S. Pat. No. 4,837,230) have worked with ceramic matrix composites, where a ceramic matrix material is applied to ceramic reinforcement fabric. If a PDC matrix material is applied to a ceramic reinforcement fabric, it can deform, separate and fracture. The chief purpose of Chen's invention is to coat the reinforcement fabric and utilize the properties of the ceramic matrix material as an adhesive and bonding agent which provides some structure. The composite created is not high density. Flaws introduced into the composite are distributed throughout its volume. In order to create a more dense and defect free article, repetitions of coating, polymerizing and pyrolyzing are required. These sequential processes compound stresses and defects.
Other inventors, such as Petrak et al (U.S. Pat. No. 5,707,471) have worked with fiber reinforced ceramic matrix composites where the fibers are coated and pyrolyzed under refractory conditions. The use of these pre-coated fibers to enhance bonding in a greater object is the subject of Petrak's work. Again, the composite created is not high density, and in order to create a more dense and defect free article, repetitions of coating, polymerizing and pyrolyzing are required.
Other inventors, such as Sapieszko et al (U.S. Pat. No. 6,521,246) create a matrix which they apply to a porous sponge. The sponge element is coated with the ceramic matrix, and the sponge itself is burned out leaving the ceramic matrix . . . which is further pyrolyzed into a ceramic object. Sapieszko acknowledges that the result can be a powder which is morphologically uniform and of small dimension. Sapieszko achieves highly and uniformly porous bodies with this approach when they don't crumble into a powder. In his approach, porosity of at least 75% is preferred.
Other inventors, such as Ostertag et al (U.S. Pat. No. 5,632,834) utilize a ceramic matrix material to bond and adhere sandwich structures. PDCs can be incorporated into the ceramic matrix, which can also include fillers.
In general, the prior art includes means of developing low density bulk ceramic from PDCs. Other prior art refers to means of creating moderate density composite structures using repeated infiltration cycles and process steps, using more infiltration process steps to increase overall density where desired and when possible.
In summary, polymer derived ceramic precursor (PDC) is a fluid which can be made (polymerized) into a plastic which can be transformed (pyrolyzed) into a ceramic.
Scaffold has been used to provide a surface for coating with PDCs, or to provide randomized gas channels for their use by functioning as gross mechanical passageways.
PDC pre-ceramic and ceramic voxels, volumetric elements, are those volumes occupied by PDC during and after polymerization and pyrolysis.
A PDC green body is formed of a collection of PDC voxels substantially interconnected with one another. In some of the aforementioned art, the majority of the body is not ceramic matrix material, and the PDC is just a binder. A PDC green body formed by a scaffold which constrains the PDC into an interconnected network of voxels would have a high volume of PDC and the majority of the body would become ceramic matrix material.
The ‘Critical Dimension’ (maximum dimension of an irregular voxel) or ‘Critical Diameter’ (maximum dimension of a spherically regular voxel) is notable, as for each PDC there is a ‘Critical Dimension’ up to which a ‘solid’ voxel can be formed without catastrophically failing during pyrolysis.
A ‘bulk’ ‘white, black or gray’ precursor, scaffold and resultant polymer green body and bulk ceramic of one constituent, can be created using a scaffold, scaffold design or lattice design to render a high density pre-ceramic and ceramic material from one of the ‘white, black or gray’ constituents.
A ‘hybrid’ white, black or gray precursor, scaffold and resultant polymer green body and fully dense and bulk ceramic of one constituent, is formed from a combination of bulk (high density), as well as fully dense material in combination in a part.
A ‘composite’ ‘white and/or black and/or gray’ precursor, scaffold and resultant polymer green body and ceramic of two or more of the ‘white, or black or gray’ constituents . . . thus having multiple constituent bulk and fully dense ceramic, and non-ceramic components; having multiple densities (e.g. high density bulk; and fully dense=hybrid characteristics) as well as multiple constituent materials.